: Understanding of the mechanisms by which nonenveloped viruses penetrate membrane bilayers during entry into cells may provide targets for novel antiviral strategies, and may also be useful for designing safer and more effective vaccine, gene-delivery and/or anti-tumor agents derived from these viruses. Mammalian orthoreoviruses are unique model systems for understanding mechanisms and regulation of membrane penetration by nonenveloped viruses. The current proposal is centered on the structures and cell-entry functions of the virion-associated outer capsid proteins mu-1, sigma-3 and sigma-1. The specific aims of the proposal are to: (1) determine the steps in outer-capsid assembly, (2) determine the steps in outer-capsid disassembly as they relate to cell entry, and determine how the mu-1 protein effects virus penetration through the cell membrane, (3) identify structural features of the outer capsid proteins as relates to their assembly and functions in entry. The proposed studies are made possible because progress has been made in re-coating subvirion particles with outer coat proteins in vitro, thereby recovering particle infectivity. These in vitro recoating methods have been adapted into a new style of molecular genetic analysis of reovirus outer-capsid structure and function by using mutant forms of recombinant proteins for recoating.